Recently, from the viewpoint of energy saving (for example, fossil energy saving) and prevention of global warming (for example, reduction of CO2 gas emissions), the improvement of efficiency (for example, the improvement of efficiency of steam turbines) of thermal power generation plants has been desired. One of the effective means for improving the efficiency of steam turbines is to increase the length of a steam turbine long blade (rotor blade). The steam turbine long blade has a problem of abrasion (erosion) of a tip portion by collision with water droplets. Erosion becomes severe because the peripheral speed of the tip portion increases as the length of the steam turbine long blade is increased. Conventionally, for the steam turbine long blade, a titanium-based alloy and an iron and steel material are used. A titanium-based material has a lower specific gravity than that of an iron and steel material, and therefore is a material suitable for increasing the length of the steam turbine long blade.
In general, in a steam turbine long blade, a forged plate having excellent abrasion resistance called “erosion shield” is bonded to a blade tip portion. As an erosion shield material for a titanium-based long blade (a long blade using a titanium-based material), a titanium-based material is used for adapting the thermal expansion coefficient, however, the titanium-based material has lower abrasion resistance than the iron and steel material, and therefore, it is difficult to further increase the length of the blade.
Therefore, in order to make a steam turbine to achieve both high efficiency and reliability, an erosion shield having excellent abrasion resistance for use in a titanium-based long blade has been demanded. As an example of a shielding method, there is a method for forming a hard ceramic coating film by vapor deposition or the like on a blade tip portion (see, for example, PTL 1).
Further, PTL 2 discloses a surface treatment method for a steam turbine rotor blade, characterized in that a blade tip leading edge portion of a steam turbine low-pressure last stage blade composed of a titanium alloy is finished into a predetermined shape by machining, and thereafter, the surface of a base material is melted by a high-energy density source, and a ceramic powder is dispersed and overlay-welded in the base material.
Further, PTL 3 discloses a method for manufacturing a corrosion resistant alloy, characterized in that a mixture composed of a ceramic powder at a volume ratio of 20 to 60% and the remainder substantially consisting of titanium or a titanium alloy powder is prepared, and thereafter, this mixture is fired in vacuum or in an inert gas, and molded by a hot isostatic pressing process.